A full-automatic horizontal clamping holder includes a rear base, a bottom support arm, a left holding arm and a right holding arm mounted to the rear base. The full-automatic horizontal clamping holder further includes a return elastic member extending vertically for returning the bottom support arm upward. A linkage assembly is provided between the bottom support arm and the left and right holding arms for linking the left and right holding arms to horizontally move inward or outward so as to clamp or release a mobile phone.